Optically active alcohols are compounds of great importance used as a synthetic raw material for the production of pharmaceuticals, agrochemicals, liquid crystal materials, fine chemicals, etc.
Among various methods for producing optically active alcohols, the most efficient is a method involving catalytic asymmetric reduction of unsymmetrical ketones, and so far a lot of catalysts for asymmetric reduction of ketones have been developed. In particular, asymmetric transfer hydrogenation using RuCl(Tsdpen)(p-cymene) or other chiral ruthenium complexes having an optically active diamine ligand, which was invented by Noyori et al. (see Patent Literature 1, for example), is the most industrially advantageous method. The reasons for this are that the product can be obtained in very high enantiomeric excess, and that organic compounds such as 2-propanol and formic acid can be used as a hydrogen source without the need of such specialized equipment as required in the case where high-pressure hydrogen gas is used as a reducing agent.
Asymmetric reduction using, as a catalyst, a complex of a transition metal, such as rhodium, ruthenium and iridium, with an optically active ligand Tsdpen (N-(2-amino-1,2-diphenylethyl)-p-toluenesulfonamide) is a conventionally well-known method, but Tsdpen is so expensive a compound as to prevent the industrial application of this reaction. This method produces optically active alcohols in good enantiomeric excess from ketones in which a carbonyl group is bound directly to an aromatic ring, but in cases where ketones in which a carbonyl group is bound to an aromatic ring via a methylene group, for example β-tetralone, are used as a starting material, the method has the serious drawback of greatly reducing the enantiomeric excess of the product (see Non Patent Literature 1, for example). For those reasons, this conventional method is not versatile and there is a pressing need for methods for industrially producing optically active alcohols.